UserLoginTestBase::assertFailedLogin in core/modules/user/tests/user.test

Make an unsuccessful login attempt.

UserLoginTestCase::testLoginMethods in core/modules/user/tests/user.test

Test that login credentials work (or not) in different login modes.

user_login in core/modules/user/user.module

Form builder; the main user login form.

user_login_authenticate_validate in core/modules/user/user.module

Second phase validation handler on the login form.

user_login_block in core/modules/user/user.module

user_login_final_validate in core/modules/user/user.module

The final validation handler on the login form.

user_login_settings in core/modules/user/user.admin.inc

Login settings form.